What causes "invalid source for %q tool: source %q is not a compatible type" in googleapis/mcp-toolbox?

Point the tool's source field at a source of kind "looker" in the config

invalid source for %q tool: source %q is not a compatible ty

Error message

invalid source for %q tool: source %q is not a compatible type

What it means

ValidateSource verifies at startup that the source bound to this lookerdeleteprojectfile tool implements the looker-compatibleSource interface. If the configured source is any type other than a Looker source, the type assertion fails and this error is returned, preventing the tool from being invoked against an incompatible backend.

Source

Thrown at internal/tools/looker/lookerdeleteprojectfile/lookerdeleteprojectfile.go:108

// validate interface
var _ tools.Tool = Tool{}

type Tool struct {
	tools.BaseTool[Config]
}

func (t Tool) GetSourceName() string {
	return t.Cfg.Source
}

func (t Tool) ToConfig() tools.ToolConfig {
	return t.Cfg
}

func (t Tool) ValidateSource(source sources.Source) error {
	_, ok := source.(compatibleSource)
	if !ok {
		return fmt.Errorf("invalid source for %q tool: source %q is not a compatible type", t.Cfg.Type, t.Cfg.Source)
	}
	return nil
}

func (t Tool) Invoke(ctx context.Context, s sources.Source, params parameters.ParamValues, accessToken tools.AccessToken) (any, util.ToolboxError) {
	source, ok := s.(compatibleSource)
	if !ok {
		return nil, util.NewClientServerError("source used is not compatible with the tool", http.StatusInternalServerError, nil)
	}
	sdk, err := source.GetLookerSDK(ctx, string(accessToken))
	if err != nil {
		return nil, util.NewClientServerError("error getting sdk", http.StatusInternalServerError, err)
	}

	mapParams := params.AsMap()
	projectId, ok := mapParams["project_id"].(string)
	if !ok {
		return nil, util.NewAgentError(fmt.Sprintf("'project_id' must be a string, got %T", mapParams["project_id"]), nil)

View on GitHub (pinned to 8cc6e09de2)

Solutions

Point the tool's source field at a source of kind "looker" in the config
Confirm the referenced source is defined and registered as looker
If calling programmatically, pass a sources/looker source instance

Example fix

// before
sources:
  db: { kind: postgres, ... }
tools:
  del-file: { kind: looker-delete-project-file, source: db }
// after
sources:
  lk: { kind: looker, ... }
tools:
  del-file: { kind: looker-delete-project-file, source: lk }

Defensive patterns

Strategy: type-guard

Validate before calling

if err := tool.ValidateSource(src); err != nil { return fmt.Errorf("config invalid: %w", err) }

Type guard

func isLookerSource(s sources.Source) bool { _, ok := s.(compatibleSource); return ok }

Try / catch

if err := tool.ValidateSource(src); err != nil { log.Fatalf("incompatible source: %v", err) }

Prevention

Call ValidateSource once at startup for every tool
Keep tool `source:` names pointing at looker-kind sources
Add config tests that load and validate all tool definitions

When it happens

Trigger: Server startup/config validation where the tool's YAML `source:` points to a non-looker source kind; calling ValidateSource directly with a foreign sources.Source.

Common situations: Copy-pasted tool configs referencing the wrong source name; refactors that renamed or re-typed a source; multi-source configs where the tool was pointed at postgres/http instead of looker.

Related errors

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/3ccea5019f483ae3. Report an issue: GitHub.